When conducting drilling operations, operators employ any number of methods and devices to ascertain information about downhole conditions. For example, information about a formation's lithology and the type of formation fluid can be determined using wireline logging or measurement-while-drilling (MWD) techniques. In developed reservoirs, similar information can be inferred from logs of offset wells, geological maps, and the like. Cuttings circulated to the surface may also be used to identify a formation's lithology, and changes in mud weight or resistivity (in water based muds) can be used to indicate the presence of hydrocarbons. Numerous other techniques and tools may also be used to gather information about downhole conditions.
Most of the methods mentioned above, however, do not provide real-time information during drilling operations. Wireline logging can be conducted only after the formation of interest has already been drilled through. Offset logs offer only general guidance, since there is no guarantee that the location, porosity, thickness, etc. of a formation of interest will be the same between offset wells and a well being drilled. Cuttings and changes in mud characteristics are known only after mud that is at the bit has had time to circulate to the surface. Some MWD tools require drilling to be suspended while a wireline or slickline is dropped through the drill string to retrieve data recorded by the downhole tool.
Acoustic-type MWD tools may provide information about formation properties in real-time. Acoustic-type tools generally measure various properties of acoustic signals (e.g. the time it takes an acoustic signal to travel from a transmitter, through the formation, and back to a receiver) to determine properties of the rock surrounding the wellbore. Such tools may be hindered by, for example, the acoustic signal being required to pass through multiple formation types having different acoustic properties, variations in mud density, noise generated by the drill string, or the like.